1. Technical Field
The present invention relates to control circuits and, more particularly, to control circuits for heaters with variable resistance associated with sensors for detecting oxygen in exhaust gases.
2. Prior Art
The present O.B.D. II standards for emissions (exhaust gases) require the operating temperature of the lambda probe to be kept within a well-defined range. This requirement cannot be satisfied simply by the selection of the resistance of the heater and/or by automatic compensation thereof.
The functionality of the heater can be monitored by accurate detection of the current passing through the measurement resistor associated with the heater and the functionality of the probe can thus be deduced for a subsequent adjustment operation, if necessary.
In view of the tight tolerances imposed by the O.B.D. II standard, the current measurement has to be particularly accurate but this conflicts with two contrasting requirements relating to the resistance of the measurement resistor.
In fact, because of the high initial current, this resistor has to have a very low resistance. In the steady state, however, when the current is lower, the voltage drop in a measurement resistor with low resistance would be a few mV giving rise to large measurement errors, which also arise because of the offset and drift which are not negligible in comparison with the useful signal.
The use of a measurement resistor with a high resistance, on the other hand, would involve a loss of efficiency of the heater at the stage immediately following activation (a longer time required to reach the steady state) and, in particular, would involve a large power dissipation by the measurement resistor during the initial stage, with obvious problems of size.